The discussion throughout this specification comes about due to the realisation of the inventor and/or the identification of certain related art problems by the inventor and, moreover, any, discussion of documents, devices, acts or knowledge in this specification is included to explain the, context of the invention. It should not be taken as an admission that any of the material forms a part of the prior art base or the common general knowledge in the relevant art in Australia or elsewhere on or before the priority date of the disclosure and claims herein.
It is to be appreciated that any discussion of documents, devices, acts or knowledge in this specification is included to explain the context of the present invention. Further, the discussion throughout this specification comes about due to the realisation of the inventor and/or the identification of certain related art problems by the inventor. Moreover, any discussion of material such as documents, devices, acts or knowledge in this specification is included to explain the context of the invention in terms of the inventor's knowledge and experience and, accordingly, any such discussion should not be taken as an admission that any of the material forms part of the prior art base or the common general knowledge in the relevant art in Australia, or elsewhere, on or before the priority date of the disclosure and claims herein.
Rock bolts have been used for many years to support earthen formations for mining and civil engineering.
Where used herein the term ‘rock bolt’ is intended to refer to any elongate anchor for location in drill holes to stabilise rock excavations and may alternately be known by names such as ‘rock stabiliser’, ‘roof-bolt’, ‘friction stabiliser’ or ‘split-set’ bolt. Rock bolts transfer load from the unstable exterior of an earthen formation, to the confined and much stronger interior of the rock mass.
Traditional rock bolts generally consist of steel rods with a mechanical or chemical anchor at one end or chemical anchor along their whole length and a face plate and nut at the other end. They are typically tensioned after installation by tightening the nut.
Another invention, well known ‘split set’ rock bolts have an elongate tube that are generally of circular or C-shaped cross-section and having a longitudinal channel or groove along the entire length of the tube. Split set rock bolts are usually installed into a hole drilled into an earthen formation using an impact tool.
With Split Sets the diameter of the drill hole is slightly less than the outer diameter of the elongate tube, so that during installation, the elongate tube is subject to radial compressive force. The compressive force causes the channel or groove to at least partly close, reducing the diameter of the tube to fit the diameter of the hole. This ensures that there is at least some frictional engagement between the elongate tube and the earthen formation.
For permanent applications, high load areas, or in rock in which corrosive groundwater is present, the space between the bolt and the surrounding earth can be filled with an aqueous slurry of cementitious grout. This keeps the rock bolt in place and reduces the likelihood of rock bolt failure due to rusting or corrosion. After grouting the bolt installations shear strength is also increased, but grout shrinkage can reduce the stiffness of the rock bolt. The grout is typically a coarse cement composition that can flow along the drill hole and into narrow cavities to fill them. The grout subsequently sets to form a solid that consolidates with the adjoining earth to form a consolidated mass.
The traditional method of grouting rock bolts is to use a short feed tube through which the grout is pumped into the drill hole, plus a smaller diameter breather tube extending to the end of the hole, to bleed the air from the hole. However, there are problems associated with the grouting of rock bolts. It can be difficult to pump grout into a drill hole in a matter that causes it to flow along the entire length of the rock bolt tube. Often air is trapped inside the drill hole and cannot be vented to make way for the flow of grout. Also, rock bolts are often inserted into the vertical drill holes in mine roofs. If grout of low viscosity or slow setting time is used it can tend to fall out of the drill hole under the action of gravity. Full mechanical coupling does not occur until the grout has set therefore, in the case of tunnels, the workspace is not supported and work cannot be performed under unsupported ground. This slows up the tunnelling work at an enormous expense.
Mechanically anchored rock bolts can work loose from the drill hole, particularly in weak, closely jointed or soft rock. In these situations chemical anchors are often used. Chemical anchors typically consist of a cartridge sheath containing two separate compartments, one holding a resin and the other holding a catalyst. The cartridges are pushed to the end of the drill hole ahead of the rock bolt, which is then spun into the cartridges. The compartments are thus broken open and the resin and catalyst are mixed together by the spinning action. Setting of the resin commences within a few seconds of being mixed with the catalyst and subsequently cures to produce a very strong and immediate anchor.
Resin cartridges are high cost and difficult to manufacture transport and store. The resin and the catalyst deteriorate at room temperature or higher so in hot environments there is a need for the product to be refrigerated and must undergo careful rotational stock control. The less expensive cementitious grout tends to be used for large volume applications, but compared with resin, grout is extremely slow to cure.
However there are several difficulties associated with use of resin cartridges, particularly practical problems associated with inserting the resin cartridges into “distant” holes and breaking the plastic sheath of the cartridges and mixing the resin and catalyst together effectively. In some instances the bolt will not break the packaging holding the catalyst but lay alongside it in the hole or the plastic holding the hardener will wrap around the bolt ‘isolating’ the bolts from the mixed resin and thereby not bonding the bolt to the rock. Without the catalyst packaging being broken the resin mastic will not harden and the bolt will be ineffective. This is known as “gloving”.
Drill holes are typically drilled into unsupported earth in mines and tunnels hence the need for the rock bolt to provide support. The unsupported earth around the rock hole in a mine or tunnel roof is associated with a risk of collapse or rockfall, therefore it is critical that workman do not work beneath an unsupported area. They are typically required to remain 5 to 6 meters from the area. However, it is difficult for the workmen to install resin in drill holes that are 5 to 6 meters removed from them. Preferably, sausage-shaped cartridges full of resin are pneumatically blown into drill holes but the process requires precise alignment of devices that are easily damaged in the underground environment. As an alternative, it is often necessary for the resin to be installed using pushing devices which are cumbersome and prone to damage. Furthermore, the resin cartridges are heavy and often fall from the drill holes. Workmen may take risks by approaching the unsupported earth to recover the fallen cartridges and by attempting to reinstall them in the drill holes. For very long holes the volume of resin that is required is quite large and the cartridges can be difficult to install into the drill holes.
Resin sausages can only be, in practical terms, in binary terms. Sometimes because variations in the drilled hole length and diameter exists, fractions of sausages re required to be installed. This is not practical resulting in poor, i.e. non complete, encapsulation of rock bolts at the very least exposing the rock bolt to corrosion.
It is also important, if not difficult, to maintain axial alignment during rotation. Furthermore the length of time and the number of rotations for spinning the rock bolt in the resin is limited. Once the setting process has been initiated, the structure of the resin can be damaged and the overall installation weakened by additional spinning. An over mixed chemical anchor is worse than an under mixed chemical anchor because the early gel formed is ‘ground up’ by the spinning.
In some rock structures, the walls of the drill hole become clay coated or wet during drilling. This tends to allow the resin cartridges to slip during rotation, resulting in incomplete mixing and unsatisfactory bonding, particularly in longer cartridges. Pockets of unmixed resin and catalyst and plastic cartridge sheath may be left attached to the drill hole wall.
In highly fractured rock structures, the resin may quickly seep into the surrounding rock so that it never comes in contact with the catalyst. This leaves voids in the resin column surrounding the rock bolt with concomitant reduction in the effectiveness of the chemical anchor as compared with fully chemically encapsulated rock bolts.
Even if the diameter of the drill hole is correct, if the hole is drilled too deeply as compared to the length of the rock bolt (ie the bolt is not ‘bottomed’ in the drill hole), the resin and catalyst cannot be mixed at the end of the hole and is completely wasted.
Furthermore, packaged resin “sausages’ are difficult and relatively expensive to manufacture.
Accordingly there is a need for better and more efficient loading of drill holes with fixatives such as grout and resin.